Preparation of diborane



Feb. 7, .1961 c. CHAPPELOW, JR. EI'AL 2,970,894

PREPARATION 9F DIBORANE Filed April 5. 1956 PARTIAL CONDENSER IOOLANT T 9 c -g{- 8'5" 4 SOEUfiON g HEAT OR FLASH EVAPORATOR EVAPORATO'R PREPARATION OF DIBORANE FROM DIALKOXYBORANES NM I. y

IN V EN TORS United States Patent i 2,970,894 PREPARATION OF DIBORANE Cecil C. Chappelow, Jr., and Roland L. Hughes, Kansas City, Mo., assignors to Callery Chemical Company, Pittsburgh, Pa., a corporation of Pennsylvania Filed Apr. 5, 1956, Ser. No. 576,470

4 Claims. (Cl. 23-204) This invention relates to an improved method for the preparation of diborane and more particularly to its preparation by the disproportionation of a dialkoxyborane [BH(OR) in the presence of a boroxine [B O (0R) solution.

Diborane (B H has become very important in recent years as an intermediate in the preparation of high energy fuels and other boron containing compounds. Recently it has been proposed that diborane could be producedusing dialkoxyboranes such as dimethoxyborane [HB(OCH as an intermediate which would disproportionate into diborane and the corresponding trialkyl borate. In Bush, Serial No. 498,735, filed April 1, 1955, and McElroy et al., Serial No. 498,737, filed April 1, 1955, there are described processes for preparing dialkoxyboranes by the reaction of an alkali metal hy dride or borohydride with a trialkyl borate or a mixture of a trialkyl borate and a trialkoxy boroxine. The processes described in these co-pending applications resulted in the production of dialkoxyboranes such as dimethoxyborane in high yield. It has been found, however, that dialkoxyboranes such as dimethoxyborane are much more stable than had been previously reported. In the prior literature (e.g. The Chemistry of the Hydrides by .D. T. Hurd) dimethoxyborane (sometimes referred to as dimethoxyborine) is described as being a very unstable compound which disproportionates to diborane and methyl borate very rapidly at room temperature. The instability of dimethoxyborane is indicated as being so great that its boiling point can be determined only by extrapolation of the vapor pressure data. Recent experimental work, however, has shown that at 40% C. :gaseous dimethoxyborane disproportionates only about .3 to 6% in 200 minutes. Similarly, at 80 C. gaseous dimethoxyborane disproportionates only about in 200 minutes. The disproportionation proceeds at a somewhat faster rate in the liquid phase. The rate of disproportionation is also increased with an increase in temperature when suflicient pressure is applied to keep the dimethoxyborane in the liquid phase, as is disclosed in the copending application of Schechter and Huff, Serial No. 510,527, filed May 23, 1955, now abandoned. The rate at which the disproportionation proceeds is relatively slow unless high temperatures (to about 80 C.) and corresponding high pressure are used. At 32% C. only about 16% of a sample of dimethoxyborane was disproportioned after minutes.

It is therefore an object of this invention to provide an improved method for preparing diborane by the disproportionation of dialkoxyboranes.

It is a further object to provide a method of rapidly disproportionating dialkoxyboranes at moderate temperatures and pressures to obtain diborane and a trialkylborate.

Another object is to provide a process to prepare diborane from the disproportionation of liquid dialkoxyboranes in the presence of trialkoxyboroxine solutions.

2,970,894 Patented Feb. 7, 1961 It is a more specific object to provide :a process to prepare diborane from the disproportionation of dimethoxy borane at moderate conditions of temperature and pressure in the presence of a methoxyboroxine solution.

Other objects will become apparent throughout the specification and claims as hereinafter related.

The attached drawing is a schematic flow sheet of a process of preparing diborane and regenerating a concentrated boroxine solution.

This invention consists of a method of effecting an in creased rate of disproportionation of dialkoxyborane to diborane and a trialkylborate which comprises contacting the dialkoxyborane with an alkoxyboroxine solution.

This invention is based on the discovery that the rate at which a dialkoxyborane disproportionates into diborane and the trialkyl borate is markedly increased when the liquid dialkoxyborane is in contact with a solution of an alkoxyboroxine. An increase in the rate of disproportionation of liquid dimethoxyborane occurs when any amount of the alkoxyboroxine is present. It is effective when using small catalytic amounts of alkoxyboroxine as well as when using a large excess of alkoxyboroxine. The rate at which diborane is produced is higher in any mixture of dialkoxyborane and alkoxyboroxine at any temperature than with dialkoxyborane alone at a corresponding temperature.

The term alkoxyboroxines throughout this application refers to any liquid whose composition can be precisely expressed in terms of B 0 and trialkylborate content. These compositions may be considered as a solution of boric oxide in the trialkyl borate in which the amount of boric oxide in solution may be varied. It is our belief, however, that the true picture is one of a continuously varying mixture of condensation products, having many different molecular weights, the exact nature depending upon the ratio of the components B 0 and B(OR) Any particular solution can be defined in that it contains only boron, oxygen and certain alkoxy groups and has a certain boron content. For example, methyl borate contains 9.62 milliatoms of boron per gram. Any solution containing only boron, oxygen and methoxy groups and containing more than 9.62 milliatoms of boron per gram is a boroxine solution. In the case of methoxyboroxines it is preferred to use a solution containing less than 20 milliatoms of boron per gram, as solutions containing higher amounts of boron are extremely viscous. The composition which is commonly referred to as trimethoxyboroxine has a boron content of 17.2 milliatoms of boron per gram.

Dimethoxyborane disproportionates according to the equation 6BH(OCH =B H +4B(OCH The methoxyboroxine solutions can be expressed as a composition of B 0 and B(0CH For convenience the composition of liquid mixtures of dimethoxyborane and methoxyboroxine solutions are expressed as concentrations of boric oxide, methyl borate and dimethoxyborane.

In a series of experiments the following described apparatus and experimental procedure was used. A 250 millititer 3-necked flask was fitted with a mercury sealed stirrer, a dropping funnel, and an insulated cold finger reflux condenser. The top of the condenser discharged into a spiral acid scrubber and from the scrubber into a wet-test meter. Dimethoxyborane was introduced into the flask and kept at a temperature of about 25 C. Methoxyboroxine solution to be added from the dropping funnel was prewarmed to a temperature such that the reacting mixture of dimethoxyborane and methoxyboroxine would have a temperature of 20 C. The prewarmed trimethoxyboroxine was admitted to the flask under a nitrogen pressure. Disproportionation started immediately. Any vaporized dimethoxyborane or methyl '"aemsas borate was returned to the reaction fiask as reflux from the cold finger condenser which was cooled with dry ice in alcohol. The diborane produced passed to the acid scrubber and reacted to form hydrogen, 6 mols of hydro- The more dilute or degraded boroxine solutions can easily be concentrated or upgraded by a simple evaporation step. The trialkyl borate is the only volatile product obtained when an alkoxyboroxine solution is heated gen for each mol of diborane. The amount of hydrogen or boiled. The upgrading evaporation can be incorporatwas measured to determine the amount of diborane proed as a regeneration cycle in a process to prepare diborane duced. The results of a series of runs at atmospheric in which there is no consumption of boroxine. pressure are set forth in Table I. This process is schematically illustrated in the attached figure, Dimethoxyborane and boroxine solutions are T able 1 brought together in the reactor 1 at super-atmospheric pressure. The dimethoXyborane can be fed as a liquid, Starting Composition Ber-cent of or fed as a gas and condensed or absorbed in the reac- Temp" tor. The pressure is reduced across valve 2 and diborane 101 mol mol 0. proportionis removed at atmospheric pressure. The pressure is DMB B203 MB also reduced across valve 3 and the liquid products are sent to an atmospheric pressure flash evaporator 4. In M79 0(]( 035.2 20 this flash evaporator any dissolved diborane and undis- 8 286 8: 8:33 38 3,3 2 proportionated dimethoxyborane are vaporized. The di- 0449 0.205 0. s11 20 37.0 methoxyborane is condensed by the partial condenser 5 M07 M88 0'557 30 20 and returned to the reactor 1. Diborane passes this parpDimethoxybomge -tial condenser and is joined with the major product stream Methyl borate. 6 from the reactor 1. The liquid product from the flash T presence of only a Small amount of B203 (as evaporator 4, a dilute boroxine solution is passed to the boroxine solution) increases the rate of diborane formaevaporator Methyl borate 5 removed f the solu' {i very i ifi l A increase of h B 0 content tion in this evaporator and the liquid bottoms product is "of the reaction mixtures increases the rate of diborane then concentrated boroxine solution. This concentrated formation. Increase in temperature also increases the boroxine solution is returned to the reactor 1 and re- 'rate of diborane formation. Boroxine solutions of any used. Dimethoxyboran'e is the only material charged concentration can be used, although it is preferred to use t th process and diborane and methyl borate are the Solutions With 3. relatively bOIlC oxide content. only products of the process process can be pe apparatus f constructed to Permit the ated continuously or batchwise as a succession of operayestigation of the reaction at superatmospheric pressure. tions' :Ihe apparatus was essentially the same as that used in other alkoxybomxines may be used equally as well as the atmospheric pressure experiments. The reactor a ethoxyboroxine to increase the rate at which 'dimethit: :23 g gg g gfi 2:2; gllagnetlc oxyborane disproportionates. Other dialkoX-yboranes r as Consuch as diet-hoxyborane also disproportionate very rapidly nected to a reflux condenser through a h1gh-pressure sight glass. A back pressure on the reactor and conh pFesence i alkoxyberoxme q Th8 procd'enser system was controlled by a Cartesian manostat. 40 655 lliustrted Wlth methfJXy boranes 3 Dibomne Produced was discharged from the manostat borox1nes,rs equally operative with other dralkoxyboranes through a wet test meter (filled with kerosene rather and alkoxyboroxlnes' than water) which measured the diborane produced. Di- While We have descrlbed our Process as Tequlred by methoxyborane was very rapidly dispr ti t d at law and have illustrated several preferred embodiments slightly elevated temperature and pressure in the presof this process it is to be understood that within the once of boroxine solutions. The results of a series of pe of pp Claims this invention may be P runs are set forth in Table II. ticed otherwise than as specifically described.

Table 11 Starting Composition Percent BFItOCHa): dls- Temp, Pressure, proportlonated after 0. p.s.i.g. mol mol mol B205 B(0OH BH(OCH )2 1min. 3min. 6min.

o 0 51 40-42 47-50 3. 3 7. 5 14. 0 25 .25 .54 42 37 40. 5 49. 0 50 .50 .54 4245 37 5a. 0 73. 5 100 .50 .50 .20 38-42 as 100 50 .50 .54 75-77 50 90. 5 100 at 2% min.

The by-product of the disproportionation of dirneth- 'oxyborane is methyl borate which is also a component of the boroxine solution. Thus the boroxine solution obtained from a dfsproportionation reaction is more dilute, or degraded, than the original solution. For example, ,when a mixture of 1 mol of dimethoxyborane is disproportionated in a boroxine solution containing 1 mol of B 0 and 1 mol of methyl borate, 0.67 mol of methyl borate is produced by the disproportionation reaction. The resulting boroxine solution then contains 1.67 mols .of methyl borate for each mol of B 0 it is preferred .to use a boroxine solution of such a concentration and in {such amounts that the final boroxine solution contains $02) more than .2 ,mols of methyl borate for each mol of What we desire to claim and secure by Letters Patent of the United States is:

1. A process for preparing diboranc which comprises disproportionating a di-lower-alkoxyborane in the presence of an lower-alkoxyboroxine, recovering the diborane formed, and evaporating tri-loweralkyl borate from the lay-product diluted boroxine solution, recovering the triloweralkyl borate, and recycling the concentrated boronine solution for the disproportionation of additional dialkoxyborane. I

2. A process according to claim 1 in which the dialkoxyborane is dimethoxyborane and the alkoxyboroxine is a methoxyboroxine.

3-. A process according to claim 2 in which. the conceat a ed boraxin eqlution is of su h ompq's i thet 5 the dilute boroxine solution has a composition equivalent to a solution containing not more than 2 mols of methyl borate for each mol of B 0 4. A method according to claim 3 in which the disproportionation reaction is carried out at a temperature 5 between 40 and 80 C. and a pressure between 35 to 50 pounds per square inch.

References Cited in 31c file of this patent UNITED STATES PATENTS Schlesinger et al. Ian. 17, 1950 OTHER REFERENCES Hurd: "Chemistry of the Hydrides, page 91, John Wiley & Sons (1952). (Copy in Scientific Library.) 

1. A PROCESS FOR PREPARING DIBORANE WHICH COMPRISES DISPROPORTIONATING A DI-LOWER-ALKOXYBORANE IN THE PRESENCE OF AN LOWER-ALKOXYBOROXINE, RECOVERING THE DIBORANE BY-PRODUCT DILUTED BOROXINE SOLUTION, RECOVERING THE TRIBY-PRODUCT DILUTED BOROXINE SOLUTION, RECOVERING THE TRILOWERALKYL BORATE, AND RECYCLING THE CONCENTRATED BOROXINE SOLUTION FOR THE DISPROPORTIONATION OF ADDITIONAL DIALKOXYBORANE. 